This paper examines the role that unions and collective bargaining play in improving working conditions in garment factories participating in the Better Work program in Bangladesh, Cambodia, Haiti, Indonesia, Jordan and Vietnam. Based on analyses of factory assessment data over repeated cycles of capacity-building and monitoring, we find that workplace unionization and collective bargaining are associated with lower non-compliance in salaries and benefits, contracts, as well as occupational safety, health, and welfare standards. Findings, however, are much less definitive for working hours. While local capacity to enforce accountability and better working conditions remains circumscribed by the business imperatives of fast fashion strategies, this study demonstrates local industrial relations systems have the potential to augment the efficacy of transnational, collaborative interventions such as Better Work.